1. Field of the Invention
The present invention relates to an illumination control circuit. More particularly, the present invention relates to a pulse width modulation (PWM) illumination control circuit with low visual noise for driving a light-emitting diode (LED).
2. Description of the Related Art
In recent years, conventional cathode ray tubes (CRT) are gradually being replaced by liquid crystal displays (LCD) due to big improvements in the semiconductor manufacturing techniques. LCD has many advantages over CRT including lower power consumption, a lighter weight, a higher resolution, higher degree of color saturation and a longer service life. For these advantages, LCD is being widely used in many electronic products including digital cameras, notebook computers, desktop monitors, mobile phones, personal digital assistants (PDA), car television, global positioning systems (GPS), palm-top game player, electronic translators and even digital watches and so on.
In general, a liquid crystal display uses an array of light-emitting diodes (LED) driven by a simple driving circuit to serve as a light source. However, due to the special properties of an LED, brightness of the LED is not linearly related to the driving current. Furthermore, color of the LED may also vary according to the driving current. Hence, for a liquid crystal display that uses LED as a back light or illumination system, difficulties are often encountered when the illumination is varied by directly adjusting the driving current.
To avoid the difficulties of illumination adjustment through an amplitude variation of the driving current, a driving current with a constant amplitude is used with the illumination adjustment achieved through a pulse width modulation (PWM) of the driving current. Ultimately, the LED is able to produce a consistent emitting efficiency within a broad range.
FIG. 1 is a block diagram of a conventional pulse width modulation illumination control circuit. FIG. 2 is a diagram showing the relationships between illumination control pulse signals and light-emitting diode driving current signals for the circuit in FIG. 1. In FIG. 1, an illumination control pulse signal Con that sets the illumination of the light-emitting diode is sent to a DC/DC converter 110 to produce a light-emitting diode driving current signal Id for driving a light-emitting diode. The waveform diagrams (a), (b) and (c) shown in FIG. 2 represent three different pulse width settings of the light-emitting diode driving current signals Id. For example, the light-emitting diode is at full illumination (100%) in FIG. 2 (a), at 20% of the full illumination in FIG. 2 (b) and at 50% of the full illumination in FIG. 2 (c).
To prevent any perceived flickering in the light-emitting diode by the human eyes, the frequency of the illumination control pulse signal Con cannot be too low, typically above 200 Hz. In other words, the illumination control pulse signal Con must operate at a sufficiently high frequency so that the human eyes can retain a visual image and yet perceive a steady change of illumination without flickering. Obviously, these control signals may be implemented using a simple switching circuit that controls the on/off states of the entire DC/DC converter.
Because the frequency and duty cycle of the aforementioned illumination control pulse signal Con is set to be fixed according to the required illumination, interference with the vertical, horizontal scanning signals may occur when used as the back light in a liquid crystal display. The difference in frequency between the back light and the video signals often leads to a so-called ‘fanning effect’, a watery wave pattern of an image on a display screen. In addition, the switching on or off of the DC/DC converter also leads to a significant loading on the power supply that provides power to the DC/DC converter. In other words, a ripple waveform synchronized with the illumination control pulse signal Con is also produced in the power supply. Once again, the ripple waveform may affect the video display signals leading to a flickering screen.
To prevent an interference between the illumination control pulse signal Con and the vertical, as well as the horizontal scanning signals due to their frequencies difference, the illumination control pulse signal Con and the horizontal scanning signals are synchronized to a frequency an integral multiple of each other. However, this arrangement will increase the production cost. To reduce the ripple waveform in the power supply, the frequency of the illumination control pulse signal Con can be increased. Yet, increasing the frequency of the pulse signal Con leads to higher power consumption. With the demand for a larger display screen and a lesser visual noise, fabricating a light-emitting diode illuminated liquid crystal display with a low noise and a broad adjustable range of illumination is increasingly difficult.